A lubrication system of a gear unit comprises typically a lubrication pump that is arranged to circulate lubricating oil through a gear stage or gear stages of the gear unit and through the bearings of the gear unit. As the viscosity of the lubricating oil is strongly dependent on temperature, the lubrication pump has to be designed and operated in way that the lubrication pump is not damaged even if the lubricating oil is cold and thereby its viscosity is relatively high.
In principle, it is possible to pre-heat the lubricating oil so that the lubricating oil is, at all circumstances, sufficiently warm to be pumped in the same way as during the normal use of the gear unit. However, the pre-heating so that the lubricating oil is also in cold start circumstances sufficiently warm to be pumped in the same way as in the normal use takes time and consumes energy before the gear unit is ready for use. Therefore, the lubrication pump is normally rotated with a smaller torque and speed in the cold start circumstances than in the normal use.
There is, however, a tradeoff between the time required for the cold start process and a fault risk and also wear of the lubrication pump so that higher upper limits of the torque and the rotational speed during the cold start process lead to a shorter cold start process but also to an increased fault risk and increased wear of the lubrication pump. In order to keep the fault risk as well as the wear of the lubrication pump at a sufficiently low level, a safety margin is needed when selecting the torque limit and the rotational speed limit to be used during the cold start process. This may, however, lead sometimes to a situation in which the temporal duration of the cold start process is unsatisfactorily long.